Air Separation by Catechol-Ligated Transition Metals: A Quantum Chemical Screening

The separation of O₂ and N₂ from air is of great importance in a variety of industrial contexts, but the primary means of accomplishing the separation is cryogenic distillation, an energy-intensive process. A material that could enable air separation to occur at conventional temperatures would be of great economic and environmental benefit. Metalated catecholates within metal-organic frameworks have been considered for other gas separations and are shown here to have significant potential for air separation. Calculations of interaction energies between catecholates with first-row transition metals and guests O₂ and N₂ were performed using density functional theory and multireference complete active space self-consistent field followed by second-order perturbation theory. A general recipe is offered for active space selection for metalated catecholate systems. The multireference results are used to rationalize O₂ binding in terms of redox activity with the metalated catecholate. O₂ is predicted to bind more strongly than N₂ for all cases except Cu²⁺, with general agreement in the binding trends among all methods.